The invention relates generally to spread-spectrum based EMI protection devices and methods and more particularly to FIFO memories with spread-spectrum protection.
Electromagnetic interference (EMI) has become a growing problem in today""s computers and other processing devices including portable Internet appliances and many other processing devices in part due to the higher operating frequencies. In addition, integrated circuits are combined on motherboards, and the additional EMI generated by differing components may have an additive effect. High speed busses, clock generators and other circuits can be the cause of EMI.
One technique for controlling EMI for a graphics processing system and other processing systems, may include the positioning of EMI containment shields over certain circuits. Also, metal housings are used to reduce the amount of EMI radiated from an integrated circuit or integrated circuit subsystem.
Alternatives to shielding techniques include pulse shaping, slew rate control, staggered outputs and spread-spectrum techniques. Examples of such techniques are described, for example, in an article entitled xe2x80x9cUse spread-spectrum techniques to reduce EMIxe2x80x9d authored by Steve Bolger and Samer Omar Darwish, Integrated Circuit Systems, published in 1998, by Reed Elsevier Inc. and incorporated herein by reference.
The U.S. Federal Communications Commission requires electronic devices to provide minimum EMI so that surrounding devices are not improperly affected. Spread-spectrum EMI reduction typically involves the use of a jitter clock that varies the frequency of data about a range to decrease the magnitude of a maximum energy at any given frequency. This is typically done by controlling the frequency of a jitter clock. Spread-spectrum clock generators can typically generate two spread-spectrum clocks, such as a center spread spectrum clock and a downspread spectrum clock based on a selection of a mode input, if desired. One type of spread-spectrum modulation sometimes referred to as xe2x80x9ccenter spread modulationxe2x80x9d shifts the frequency of a data above and below an original fundamental frequency. One advantage of this modulation is that the average system frequency always be the same. Another technique, sometimes referred to as xe2x80x9cdownspread modulationxe2x80x9d, requires a shifting of the carrier frequency down by half of the modulation amount so that the modulate away form is centered on the new carrier. Accordingly, the peak of the modulation is at the original carrier level. One advantage of downspreading is that is can insure that a system does not exceed the maximum processor""s clock speed.
In graphics processing systems, it is known to use spread spectrum EMI control for digital information that is provided to a digital display. Typically, if two display devices can be driven by a graphics processing system, one display engine path is typically used to provide information to an analog display device whereas another display engine path is used to provide display information for a digital device, such as an LCD display device. The two display engines can be used to display data simultaneously on two different displays. For example, one display engine can display graphics and video on an LCD display whereas a different image, such as a television program, can be displayed on an analog computer screen. The display information for the LCD display device may be sent using spread-spectrum EMI techniques which jitter the clock to the display engine for the display device. However, a problem arises since there are typically two display engines required for two different types of displays. Accordingly, such graphics processing systems can be quite costly. For example, one RAMDAC may be used for each processing path since the bit rates can be different. In addition, if one of the display engines has less functionality than another of the display engines, the graphics processing system can be limited in operation. For example, if only one engine can operate in VGA mode, the same image cannot be shown on two displays. This splitting of display engine capabilities can limit the operational capabilities of the processing system.
Accordingly, a need exists for a graphics processing system and method that provides spread-spectrum FIFO memory to provide a more efficient graphics processing system, or other type of information processing system.